1. Field of the Invention
The present invention generally relates to the input stage of an operational amplifier, and more specifically to a compensation circuit for maintaining amplifier performance for inputs outside the common mode range.
2. Description of the Related Art
The input stage for an operational amplifier includes a pair of differentially connected transistors that receive differential input signals and provide corresponding differential currents to an active load. The phase of the input signals or currents is a relative term and is defined by the polarity of one signal or current with respect to the other signal or current. An amplifier is in phase when the phases of the input signals and differential currents are the same. Phase reversal of an amplifier occurs when the phase of the differential currents is opposite the phase of the input signals. The amplifier operates over a Common Mode Range (CMR) of input signals that lie between the high and low supply voltages. In the case of a single-sided supply, the CMR includes one of the supply voltages, typically the low supply which is commonly ground reference potential. A particular input stage for a single-sided supply operational amplifier is described in Bowers, "A 20 .mu.W Precision OperationalAmplifier", IEEE Journal of SolidState Circuits, Vol. SC-22, No. 3, June 1987, pp. 322-329.
If one of the input signals is driven outside of the CMR due to noise or improper drive circuitry, the corresponding differential transistor will either turn off or form a forward biased parasitic diode. When the junction forward biases, the phase of the differential currents reverses with respect to the phase of the input signals and may cause the amplifier to malfunction and latch up (stop working). In Analog Devices products OP-113, OP-213 and OP-413 a pair of phase compensation diodes are cross-coupled between the differential inputs and the other differential transistors' collectors to prevent phase reversal of the amplifier. When either one of the differential transistors' collector-base junctions is forward biased, the corresponding cross-coupled diode conducts and prevents a phase reversal of the differential currents. This prevention of phase reversal is at the cost of greatly increasing the magnitudes of the differential currents, on the order of twenty to thirty times the normal current values. Although amplifier failure is less likely to occur, these extremely high current levels can still cause the active load to malfunction and latch up the amplifier.